One of the most exciting frontiers in medicine is the potential use of embryonic stem cells (ES cells) for treating a host of congenital, developmental, or degenerative diseases. Cell replacement strategies are particularly relevant for tissues and organs that have little capacity for self-repair.
ES cells possess two properties that make them especially well suited for cell therapy. First, they retain the flexibility to become any one of the more than 200 cell types that make up the human body. Given the right combination of signals, ES cells develop into mature cells that can function as neurons, muscle, bone, blood or other cell types. Stem cells with such flexibility are described as "pluripotent," to indicate their high potential to differentiate into a wide variety of cell types.
A second feature of embryonic stem cells is their ability to remain in an undifferentiated state and to divide indefinitely. This property of "self-renewal" means that virtually unlimited numbers of well-defined, genetically characterized cells can be produced in culture.
Stem cells have the remarkable potential to develop into many different cell types in the body during early life and growth. Human ES cells were first isolated and cultured in 1998 by James Thomson at the University of Wisconsin. Since then, numerous laboratories have been actively engaged in identifying the optimal conditions for regulating stem cell behavior and activity. Researchers have also identified conditions that allow some specialized adult cells to be "reprogrammed" to assume a stem cell-like state.
Although the transplantation of mature cells derived from ES cells offers hope for the treatment of many diseases, obstacles to this therapy remain. One side effect that worries most researchers is the ability of undifferentiated stem cells to grow unchecked and produce tumors. Because pluripotency and self-renewal are two key features of embryonic stem cells, they can develop growths known as teratomas that bear cells of ectodermal, mesodermal, and endodermal origins. Researchers reason that if they separate the undifferentiated embryonic stem cells from their differentiated progeny, and only transplant the differentiated cells needed by the patient, it will be possible to treat the disease without the fear of tumor formation.
Stem cells have other applications in addition to producing cells for transplantation therapies. Human stem cells can serve as model systems for understanding the biology of human development, and consequently diseases that arise from faulty development. Cells derived from human stem cells can also be used for drug discovery and for testing the toxicity of different drugs and compounds. Because these are human cells, they bear human receptors and signaling and metabolic pathways. They are therefore more suitable as test models than cells derived from animals, and will provide more reliable information regarding toxicity, dosage, and metabolism of drugs that are ultimately intended for human patients.
Textbook Reference: Concept 14.1 Development Involves Distinct but Overlapping Processes